Patent
1987-10-15
1989-03-21
Miller, Stanley D.
350320, 350347V, G02F 113
Patent
active
048137715
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of Invention
This application relates to liquid crystal electro-optical switching devices and, more particularly, relates to electro-optical switching devices employing ferroelectric liquid crystals.
2. Description of the Prior Art
The manipulation of light (electromagnetic radiation in the infrared, visible, and ultraviolet spectral regions) is becoming an important information handling technology. The construction of integrated optical circuits and the transmission of information along optical fibers are particularly attractive means for exploiting the information carrying capacity of light. A variety of devices, particularly for the generation and detection of intensity modulated light, have been developed for fiber and integrated optic systems. However, the development of complete optical systems has been hindered by the lack of suitable electro-optic (EO) switches for manipulating incident light by the application of electrical signals. Prior art EO switches are made from any material by which the intensity, polarization, or direction of the light can be electrically controlled, but the ease with which the molecular orientation and hence refractive index of liquid crystals (LCs) can be manipulated by applied electric fields has led to the development of liquid crystal electro-optic switching devices which exhibit improved light manipulating characteristics.
These electro-optic switching devices of the prior art have exploited a variety of electrically-induced molecular reorientation effects in nematic and chiral smectic liquid crystal phases, as will be described below.
The electro-optic effects in LCs result from electrical and optical anisotropies of the LC phases which in turn result from molecular ordering. In nematic liquid crystals (hereinafter referred to as nematics), the molecules tend to orient so that their average symmetry axes are parallel to a locally common direction. This direction defines the unit director field n,. (Note that n is commonly written herein and in the art as a vector, but since there is no physical significance attached to its sign, that is n, and -n, describe the same physical states, n, can be represented as a line segment.) Besides this orientational order, nematics are much like ordinary liquids. A consequence of this orientational order is anisotropy of the dielectric properties of nematics. Namely, although a linear relationship D=.epsilon.E still exists between the electric and displacement fields within a nematic, the dielectric constant .epsilon. is a second rank tensor. This causes the free energy of a nematic in an externally applied electric field to depend on its director orientation. If the director is not otherwise constrained, it will rotate to the orientation that minimizes the nematic's electrical free energy. Since the dielectric anisotropy extends to optical frequencies, this electrically-induced reorientation produces an electro-optic effect that may be exploited in practical light modulation devices.
The usual method for applying electric fields in these devices is to place the LC between closely-spaced parallel electrode plates. In this geometry, voltage applied across the plates produces an electric field perpendicular to the plates. Since the electrostatic energy ##EQU1## does not depend on the sign of E, this geometry allows only one field-preferred optical state. This state will have parallel and perpendicular to n, respectively.
Since to be useful a device must have more than one optical state, some means must be found to prefer a director orientation other than the one preferred by the field. This is usually accomplished by treating the surface of the electrode plates such that they prefer a different orientation. Then, the applied field produces elastic strain in the orientation of n. When the electric field is removed, the stress resulting from that strain causes the orientation to relax back to that preferred by the surfaces. This means of operation has several consequences for the dynamic characteris
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Clark Noel A.
Handschy Mark A.
Displaytech Incorporated
Gallivan Richard
Miller Stanley D.
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